Over 28 million musculoskeletal injuries are treated annually in the U.S. including 2 million fracture fixation surgeries. Of these, tibia fractures are the most common long bone fracture. Unfortunately, such fractures are frequently associated with complications (delayed union, non-union, and infection), particularly for severe trauma such as is often sustained in combat. For instance, non-union is a significant complication (approximately 100,000 injuries, 5% of all fixation surgeries in the U.S. go on to non-union), with even higher rates for severe trauma. Infection at the site of orthopedic surgery is an on-going issue and, while the incidence has been reduced due to improvements in both surgical and post-operative procedures, its prevalence is still unacceptably high. Such complications can lead to long-term or even permanent disability or death and are responsible for significant direct and indirect health care costs.
While a variety of supporting implants and adjunct therapies are available to care givers, a crucial issue leading to complication is the inability to directly evaluate health and healing of the local area. Physicians routinely acquire X-ray images as part of diagnosis and evaluation and while these images can show the hardware and fracture callus, they do not measure mechanical properties of the fracture and cannot detect early-stage infection. 3-dimensional CT images are better at indicating bone density and determining if union has occurred, but are expensive, expose the patient to significant radiation doses (typically around 300× more than a standard X-ray), and are imperfect, especially when allografts are used. For some fracture types, dynamic X-ray images can be acquired to measure bone motion with and without external load in order to assess fracture stability. In practice however this is highly challenging. For example, even in spine fusion, where spinal processes are clearly evident and can move significantly, it has been determined that inter-observer variation of spinous process movement can be about 1.5 mm, with differences as large as 3.5 mm, compared to a recommended bone fusion cutoff criteria of less than 1 mm.
When considering localized infection, external visualization techniques provide little or no clues, particular in early stages of infection. Unfortunately, infection at orthopedic trauma sites are generally not diagnosed until after the infection has spread and symptoms have become systemic.
The lack of widely applicable tests to assess bone health such as load bearing state and early stage infection presents a major challenge for physicians and patients. Infection at implant sites can require additional surgery or even become life-threatening when diagnosis is delayed. Weight bearing before the fracture callus is sufficiently strong carries risk of re-fracture and/or hardware failure. On the other hand, unnecessary delay in weight bearing can hamper rehabilitation and is highly costly in terms of lost days of activity. Studies have shown that when the fractured bone has at least 25% of the bending stiffness of intact bone, weight bearing rarely leads to re-fracture or hardware failure. For externally fixed devices, percutaneous pins can be directly loaded to assess stiffness. When testing is carried out and this 25% threshold is used, the majority of patients begin weight bearing an average of 2.3 weeks earlier than average. Load testing on externally fixated devices can likewise identify patients with delayed and non-union for weight bearing restrictions and additional interventions.
Most orthopedic surgeries involve internal fixation, which require either a percutaneously connected gauge or remote measurements to assess load bearing capabilities during healing. A percutaneously connected strain gauge is impractical for patients and presents a number of safety challenges. A variety of remote interrogation methods based upon implanted wireless devices, ultrasound, vibrational analysis, and other approaches have been examined for non-invasive measurement of strain on orthopedic implants, but these generally require significant development as well as equipment and/or expertise currently unavailable to most care givers.
What are needed in the art are passive implantable sensors for use in conjunction with orthopedic implants that can be easily read by conventional non-invasive methods to assess local conditions and bone health at the local site. In particular, what are needed are passive sensors capable of assessing strain under load and/or early stage signs of infection and thereby to determine a current state of bone health. For instance, a passive sensor locatable on bone fixation devices that can assess health and healing in the local area of an orthopedic implant by use of conventional radiography methods would be of great benefit.